1. Field of the Invention
The present invention relates generally to a tension servo apparatus, and is directed more particularly to a mechanical tension servo apparatus for stabilizing the transportation of a tape used in, for example, a magnetic recording and/or reproducing apparatus and so on.
2. Description of the Prior Art
In a magnetic recording and/or reproducing apparatus there has been employed a so-called mechanical tension servo system which detects the tape tension, varies the mechanical braking force of a reel system and automatically makes the tape tension constant.
The prior art mechanical tension servo system uses a tension arm, which may be rotated in response to the variation of tape tension, and a brake member which may apply mechanical braking force to a reel system. In the case that the brake member consists of a brake element or piece of small size, the brake piece is attached to the tip end of the tension arm, while in the case that the brake member is formed as a band brake, one end of the band brake is attached to the tip end of the tension arm and the other end of the former is fixed to a base plate.
The above described prior art mechanical tension servo system, however, can not be free from various defects. One of the defects is that due to the inertial mass of the tension arm the phase of the operation of the tension arm is delayed for the variation of high frequencies, the variation of braking force is also delayed, and an oscillation is apt to be caused. To avoid this defect there has been proposed such a method that the braking force is applied to the brake element through viscous-elastic material to decrease the gain at high frequency where the oscillation is apt to be caused and to increase a spare phase.
In general, the friction coefficient of the brake member is changed much in accordance with temperature, moisture, using time, quality of material and so on, so that even if the servo operation is stable under a special condition as in the prior art, there will occur such a defect that due to variations of the friction coefficient the loop gain is changed much and becomes unstable or the servo system becomes less in gain.
If a condition stable for all states is set, it is only possible that the gain is very low totally under an ordinary state or the servo action is applied at very low frequency under the ordinary state. In this case, no servo action is applied at the frequencies of wow, flutter and so on and hence good servo action can not be obtained.
Another defect of the prior art is caused by the fact that reels whose diameter is changed from 3 inches to 10 inches are used. Due to the variation of the diameter, the inertial mass becomes different and also the mean value of diameters of tape roll becomes, of course, different. Thus, with the prior art stabilizing method in which the braking force is applied through only the viscous-elastic material, the gain is increased and the response frequency in the loop of the servo system is widened in the case that a reel of small size is used, so that the state is apt to be made very unstable. On the contrary, when a reel of large size is used, the response frequency becomes low and the servo action can not follow the tension variation caused by the scattering of a tape roll and the like with the result that wows occur.